As is well known, modern wireless communication devices, such as mobile handset including a CDMA cell phone, are held to ever-higher performance standards. Transmissions must be clear and undistorted, and the battery in the devices must be small and have a long life. In order to meet these consumer requirements, wireless telephone designers have moved away from using traditional silicon-BASED bipolar transistors as power amplifiers and toward using more exotic transistors, such as heterojunction bipolar transistors (HBTs). Such HBTs provide outstanding power efficiency and high linearity, thus making CDMA cell phone achieve longer battery life and better signal characteristics for voice and data.
Of course, an HBT like a bipolar junction transistor (BJT) requires a direct-current (DC) bias signal to be applied to its input terminal to establish its operating point. The operating point of a transistor may be defined as a point on a transistor's characteristic curve at which the transistor will operate in the absence of an input signal. Since changes in the DC bias signal affect the operating point of the HBT and thus adversely affect the linearity of the amplifier, the DC bias signal must be very stable and unaffected by variations in temperature or in a reference voltage Vref.
FIG. 1 illustrates a conventional power amplifier module 100 for use in a CDMA cell phone. The power amplifier module 100 includes a conventional temperature compensated bias circuit in addition to an amplifying circuit. The amplifying circuit includes an amplifying transistor Q1 having an emitter grounded; an inductor L, one end thereof being supplied with Vcc and the other end thereof being connected to a collector of Q1; an output capacitor Co disposed between the collector of Q1 and an RF_OUT terminal; and an input capacitor Ci coupled between an RF_IN terminal and a base of Q1.
The bias circuit includes a bias transistor Q2, a collector thereof being supplied with Vcc; a diode-connected transistor D1 (i.e., a bipolar transistor with short-circuited collector and base), an anode thereof being connected to a base of Q2; an additional diode-connected transistor D2, an anode thereof being connected to a cathode of D1 and a cathode thereof being grounded; and a resistor R1, one end thereof being supplied with the reference voltage Vref and the other end thereof being connected to the anode of D1.
Referring to FIG. 1, the bias circuit is used to set an operating current for the power amplifier Q1. A reference current Iref flowing from the reference voltage Vref to a circuit ground through the resistor R1 and the diode-connected transistors D1 and D2 is mirrored as a collector current Ic through the power amplifier Q1 between the supply voltage Vcc and ground. The diode-connected transistors D1 and D2 provide a compensating effect that can protect the power amplifiers Q1 and Q2 against thermal runaway due to a temperature increase thereof.
Once the reference voltage Vref is set to have a predetermined value, a bias current IB of Q1, i.e., a DC component of a base current of Q1 is fixed. That is to say, the bias circuit supplies a constant bias current regardless of the output power, which in turn gives rise to a constant quiescent current IC (i.e., a DC component of the collector current of Q1), IC being an operation current of Q1.
However, the conventional power amplifier module 100 described above is highly sensitive to variation in the reference voltage Vref. For example, if the reference voltage Vref increases, a current at the base of the transistor Q2 and subsequently a current at the emitter thereof also increase. As a result, the amount of current IB flowing into the base of transistor Q1 correspondingly increases. Inversely, if the Vref decreases, a current at the base of the transistor Q2 and subsequently a current at the emitter thereof also decrease and thus the amount of the bias current IB correspondingly decreases.
Therefore, the conventional power amplifier module 100 has drawback due to the fluctuations in the reference voltage that substantially makes the operation current Ic of the transistor Q1 fluctuate.
On the other hand, as temperature rises, respective turn-on voltages (VBE1 and VBE2) of transistors Q1 and Q2 are reduced. If the VBE1 and VBE2 are lowered, voltage VA at node A is lowered and thus a reference current Iref increases. An increment ΔIref of the reference current Iref is divided into the diode-connected transistors D1 and D2 and the transistors Q1 and Q2 at node A. As a result, a base current of the transistor Q1 increases by a portion of the increment ΔIref to thereby increase the bias current IB.
On the contrary, as temperature is lowered, respective turn-on voltages (VBE1 and VBE2) of transistors Q1 and Q2 are increased. If the VBE1 and VBE2 are increased, the voltage VA at node A is increased and thus the reference current Iref is reduced. A decrement ΔIref of the reference current Iref is divided into the diode-connected transistors D1 and D2 and the transistors Q1 and Q2 at node VA. As a result, the base current of the transistor Q1 decreases by a portion of the decrement ΔIref to thereby reduce the bias current IB.
As described above, the conventional power amplifier module 100 compensates a portion of the increment or the decrement in the bias current IB due to the variations in temperature, but the compensation result is not so much.